In the related art, as a frame structure for suspending an engine for a motorcycle, there has been proposed a frame structure which is composed of hollow cast products so as to be so strong that it can support the weight of an engine while suppressing the weight of the motorcycle body frame (see Patent Document 1, for instance). In the frame structure disclosed in Patent Document 1, the motorcycle body frame is cast in aluminum, and in the casting process, in order to achieve a hollow structure, a sand core is set inside the mold.
Also, there has been proposed a frame structure in which the cross section of the frame is opened, for the purpose of reducing the weight of the motorcycle body frame and simplifying the mold (see Patent Document 2, for instance). In the frame structure disclosed in Patent Document 2, the cross section of the frame is opened. Therefore, it is possible to cast the corresponding frame structure without using any core.
Patent Document 1: Japanese Patent Application Publication No. 2007-144500A
Patent Document 2: Japanese Patent Application Publication No. 2003-170882A
According to the frame structure disclosed in Patent Document 2, it is possible to reduce the weight of the whole of the motorcycle body frame and simply the mold, resulting in a reduction in the cost. However, since the cross section of a portion of the motorcycle body frame is opened in a width direction of the motorcycle, there is a problem that the balance between strength and rigidity required for the motorcycle body frame of the vehicle is not achieved.